The present invention relates to a thermal transfer image receiving sheet and more particularly to a thermal transfer image receiving sheet capable of forming a record image excellent in the color density, sharpness and various types of fastness, particularly durability such as fingerprint resistance and plasticizer resistance.
Various thermal transfer printing processes are known in the art. One of them is a transfer printing process which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as a polyester film, to form a thermal transfer sheet and forming various full color images on an image-receiving sheet dyeable with a sublimable dye, for example, an image-receiving sheet comprising paper, a plastic film or the like and, formed thereon, a dye-receiving layer.
In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the image-receiving material, thereby reproducing a full color image of an original by means of the multicolor dots.
Since the color material used is a dye, the image thus formed is very sharp and highly transparent, so that the resultant image is excellent in the reproducibility and gradation of intermediate colors. Therefore, according to this method, the quality of the image is the same as that of an image formed by the conventional offset printing and gravure printing, and it is possible to form an image having a high quality comparable to a full color photographic image.
Not only the construction of the thermal transfer sheet but also the construction of an image-receiving sheet for forming an image are important for usefully practicing the above-described thermal transfer process.
For example, Japanese Patent Laid-Open Publication Nos. 169370/1982, 207250/1982, 25793/1985, 64899/1985 and 82791/1988, etc. disclose prior art techniques applicable to the above-described thermal transfer image-receiving sheet, wherein the dye-receiving layer is formed by using vinyl resins such as a polyester resin, a polyvinyl chloride, a polycarbonate resin, a polyvinyl butyral resin, an acrylic resin, a cellulosic resin, an olefin resin and a polystyrene resin, or by using these resins in combination with a colloidal silica.
In the above-described thermal transfer image receiving sheet, the dye-receiving sheet is usually formed by dissolving the above-described resin in a high volatile organic solvent, for example, a general-purpose organic solvent, such as toluene, methyl ethyl ketone or ethyl acetate, to prepare a coating solution, coating the coating solution on the surface of a substrate sheet and drying the resultant coating. In this case, since the solvent is volatile, the resultant coating can be easily dried. Further, since the resin constituting the dye receiving layer, as such, is substantially lipophilic, the dyeability with a dye of the dye-receiving layer is so good that it is possible to form an image having high density and sharpness. The above-described thermal transfer image receiving sheet, however, has problems, such as fading of the formed image due to sweat or sebum migrated to the image surface when the hand touched the dye-receiving layer at its dye image portion formed by dyeing and swelling or cracking of the image-receiving layer per se, that is, a problem of fingerprint resistance, bleeding of the dye when the dye in contact with a substance containing a plasticizer, such as an eraser or a soft vinyl chloride resin, that is, a problem of plasticizer resistance, and a problem of the releasability of the thermal transfer sheet at the time of the formation of an image.
Accordingly, an object of the present invention is to provide a thermal transfer image receiving sheet which can provide an image having high density and sharpness and excellent in various types of fastness, particularly fingerprint resistance, plasticizer resistance, releasability, etc., according to a thermal transfer printing process wherein use is made of a sublimable dye.